Low-impedance tissue-device interface using homogeneously conductive hydrogels chemically bonded to stretchable bioelectronics

Stretchable bioelectronics has notably contributed to the advancement of continuous health monitoring and point-of-care type health care. However, microscale nonconformal contact and locally dehydrated interface limit performance, especially in dynamic environments. Therefore, hydrogels can be a promising interfacial material for the stretchable bioelectronics due to their unique advantages including tissue-like softness, water-rich property, and biocompatibility. However, there are still practical challenges in terms of their electrical performance, material homogeneity, and monolithic integration with stretchable devices. Here, we report the synthesis of a homogeneously conductive polyacrylamide hydrogel with an exceptionally low impedance (~21 ohms) and a reasonably high conductivity (~24 S/cm) by incorporating polyaniline-decorated poly(3,4-ethylenedioxythiophene:polystyrene). We also establish robust adhesion (interfacial toughness: ~296.7 J/m2) and reliable integration between the conductive hydrogel and the stretchable device through on-device polymerization as well as covalent and hydrogen bonding. These strategies enable the fabrication of a stretchable multichannel sensor array for the high-quality on-skin impedance and pH measurements under in vitro and in vivo circumstances.


Fig. S2 .
Fig. S2.Hydrogel homogeneity analysis.(A and B) XRM images of the freeze dried PEDOT:PSS-PAAm (A) and PEDOT:PSS-PANi-PAAm (B).(C) Vertical quantity of voids within the PEDOT:PSS-PAAm and PEDOT:PSS-PANi-PAAm. (D and E) Longitudinally cutting the PEDOT:PSS-PANi-PAAm hydrogel (D) and the conductivity variation of each sample resulting from this vertical cutting (E).(F and G) Cutting the PEDOT:PSS-PANi-PAAm hydrogel in the thickness direction (F) and the conductivity variation of each layer resulting from this vertical cutting (G).

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Fig. S3.Electrical conductivity and impedance of hydrogels functionalized with different aniline concentration.(A and B) Electrical conductivity of PEDOT:PSS-PANi-PAAm (A) and PANi-PAAm hydrogels (B) prepared by varying precursor ratio of aniline (mL) to acrylamide (g).(C) Impedance of PEDOT:PSS-PANi-PAAm hydrogel, prepared by varying aniline concentrations, as a function of frequency.The numbers indicate the precursor ratio of aniline (mL) to acrylamide (g).

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Fig. S8.Electrical and mechanical properties of PEDOT:PSS-PANi-PAAm hydrogel in response to changes in its swelling level.

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Fig. S11.Reactivity of metals in acidic condition.(A and B) 3D atomic force microscopy images (i, ii) and surface roughness profiles (iii) of Al (A) and Ti (B).The upper images (i) correspond to the as-deposited metals, while the lower images (ii) correspond to the metal thin-films immersed in a 1N HCl bath for 6 h.

Fig. S13 .
Fig. S13.Optimization of hydrogel precursor ratios for strong bonding and stability of hydrogels in acidic environment.(A) Optimization of PAAm and PEDOT:PSS-PANi-PAAm hydrogels via adjustments in the acrylamide to MBAA precursor ratio.(B) Evaluation of hydrogel stability under acidic conditions (1N HCl) relative to immersion duration.

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Fig. S15.Hydrogen bonding between a TA molecule and others including PEDOT:PSS, PANi, and PAAm within the conductive hydrogel.

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Fig. S17.Preparation steps for artificial skin.(A and B) Artificial skin made of hydrogel with different ionic concentrations (A) and pH levels (B).

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Fig. S19.Measurement of pH using multichannel patch.(A) Measured potential between PANi-PAAm hydrogel and Ag/AgCl electrode.(B) Calibration curves.(C) Measurement points of pH.(D) pH levels at the positions.

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Fig. S20.Impedance mapping during stretching deformation.(A) Images of the stretchable multichannel sensor array on the artificial tissue at various stretching levels.(B to D) Impedance mapping at different points during stretching cycles: position a-b (B), position b-c (D), and position c-d (D).

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Fig. S21.Biocompatibility of the conductive hydrogel.(A) Cell proliferation results up to 72 h.(B) Cell viability results up to 72h.

Fig. S22 .
Fig. S22.In vivo impedance and pH measurement demonstration.(A) Skin burn wound models of control, mild burn, and severe burn.The images show skin surface (left), and hematoxylin and eosin (H&E)-stained (middle) and Masson's trichrome-stained cross-sections of rat skin samples (right).(B) In vivo measurement of skin impedance in rat after skin burn injury.(C) Images of the stomach and the inserted pH sensor (left), as well as the pH sensor with a PANi-PAAm hydrogel electrode and an Ag/AgCl electrode (right).(D) In vivo measurement of gastric fluid pH in a living rat.